FREE SPIRIT: Linda Isobel Arnold had two sons and three grandchildren.

A structural engineer broke down as he described a design flaw that led to a concrete panel falling off a central Christchurch building and killing a woman in last February's earthquake.

Dick Cusiel, of LSC Consulting, yesterday gave evidence at the Canterbury earthquakes royal commission hearing into the building at 43 Lichfield St.

The building, known as the Anderson building, was one of four comprising Ballantynes department store. It had seven storeys, with the top four being a car park.

Cusiel led LSC Consulting's design of the building, which was constructed in 2002.

Linda Arnold, 57, was sitting in her car outside the building on February 22, 2011 when she was hit by a falling concrete panel and killed.

Cusiel told the commission the concrete panel, known as a spandrel, was attached to the exterior of the car park, but was not attached properly.

"It is, regrettably, this omission which in my view has contributed, with the significant force of the earthquake, to the spandrel falling away from the structure," he said.

The design was drawn by another LSC staff member, but Cusiel missed the error signed off as part of the building consent application.

Cusiel broke down as he "absolutely" accepted responsibility for the mistake.

"Having sent the consent for the drawings and noted the omission, [Linda Arnold's death] has weighed very heavily on me and will always," he said.

"I extend my very sincere condolences to the family of the victims for this extreme tragedy."

Arnold's family did not wish to comment on the hearing.

The spandrels on the Lichfield St side of the building were attached by a concrete insert known as a TCM-20, Cusiel said, but should also have had pre-cast steel ties to the floor topping, as on other parts of the building.

"The panels were affixed with weld plates and angle cleats to the columns. However, they are primarily there for the purpose of construction, to put the panels in place while the floor topping was poured.

"They were unlikely to have been sufficient to keep the panels in place in the event of a major earthquake."

In acknowledging his mistake, he said the oversight should have been picked up by Christchurch City Council staff assessing the drawings included in the building consent application.

Duncan Laing, for the council, suggested the omission was not so obvious it should have been picked up by council staff."The council would inspect the drawings, pick up any anomalies and refer them back to the engineer to discuss or change."

Cusiel replied: "If the council had inspected it they almost certainly would have said there's something amiss.

"I'm not trying to blame the council.

"I have accepted full responsibility for this. There's no need to hammer the point really."

Council environmental policy and approvals manager Steve McCarthy said the oversight had not been immediately clear.

"It wasn't obvious to our engineers, otherwise they would have picked it up.